The present invention concerns a method of protecting the transmission of cells in a telecommunication system, notably a broadband telecommunication system, such as the so-called broadband ISDN broadband network designed to transmit ATM (Asynchronous Transfer Mode) cells. More precisely, the method of the invention relates to the Transmission Convergence (TC) sub-layer of any ATM transmission system and can apply, in particular, both to the so-called SONET/SDH systems and to the so-called cell-based systems such as those described in ITU-T Recommendation I.432.
It should be stated that the Protocol Reference Model PRM of the broadband ISDN provides a physical layer which is itself subdivided into two sub-layers which are respectively the physical medium PM sub-layer and the transmission convergence TC sub-layer. The PM sub-layer includes the functions which depend only on the physical medium used. As for the TC sub-layer, this covers the functions for frame generation and recovery, adaptation to the transmission frame, delimiting the cells, generation of the so-called HEC (Header Error Control) error control code, and the transmission speed adaptation functions. Thus, the transmission convergence TC sub-layer provides, on the transmitter side, the binary functions necessary for transmitting the ATM cells on the physical medium and, on the receiver side, the functions necessary for extracting the ATM cells from the received binary stream.
In the networks, since the physical medium can be subject to various perturbations, it is possible that the ATM cells are erroneous on reception and therefore have to be destroyed. In order to reduce the rate of loss, and increase the quality of the transmission, a method has already been proposed which consists of doubling the physical medium, that is to say using two transmission links instead of only one. Such a method is now described in relation to FIG. 1.
FIG. 1 shows a transmitter 10 whose Transmission Convergence sub-layer TC implements the functions necessary for transmitting the ATM cells in a binary stream simultaneously on two links 1 and 2. It also shows a receiver 20 whose sub-layer TC normally extracts the cells from the first link 1 but, in the event of unavailability of the first link 1, switches to the second link 2 in order to extract the cells therefrom.
It should be noted that the function for determining errors in the cells carried by the links 1 and 2 and the switching function are taken care of by the convergence sub-layer TC of the receiver 20.
The drawback of this method is essentially connected with the switching between the two links 1 and 2 which is not instantaneous, which leads to the loss of data during the switching time.
A method proposed at the IEICE conference on Mar. 27 to 30, 1995 and presented by Hiroshi Ohta and Hitoshi Uematsu consists in that the Transmission Convergence (TC) sub-layer of the transmitter implements the functions necessary for transmitting the ATM cells simultaneously on two links, and in that two TC sub-layers of the receiver respectively implement the functions for extracting the ATM cells from the data received from the two links. On account of the main and secondary links a priori having different lengths, a mechanism is provided in the receiver to compensate for the difference in transmission time between the flows issuing from the two links, and thus resynchronize these flows of ATM cells. When the receiver detects an incorrect cell on the main link, the mechanism then switches to the secondary link. But unlike the previous case, and in order to solve the problem mentioned above, the switching takes place on the cell preceding this erroneous cell, so that there is no switching delay. It should be noted that the same rule applies to the secondary link.
This prior method has the advantage of implementing the switching between two transmission links with no loss of ATM cells. This is because the ATM cells received on each link are phased to compensate for the difference in transmission time between the two links, and as soon as the system detects errors on one of the links, it goes over to the other, settling on the cell which precedes the one where the error was detected.
Nevertheless, when the receiver detects an erroneous cell on one of the links, it switches to the second, but nothing guarantees that the latter is actually available.
The aim of the invention is therefore to propose a method of protecting the transmission of cells in a protecting telecommunication system such as just described but which does not have the drawback thereof mentioned above.
To do this, the said method consists, on the transmitter side, of inserting, regularly in each of the said flows, cells serving as markers and thus delimiting blocks of cells or sets of blocks of cells, and, on the receiver side, of selecting, block after block or group of blocks after group of blocks, the block or group of blocks from the flow of cells which has the fewer transmission errors compared with the block or group of blocks of the same order number in the other flow.
According to another characteristic of the invention, it consists of delimiting blocks or sets of blocks each having a physical cell containing information concerning the errors in the said block or in each of the said blocks of the said set, and in that it consists of selecting the block or group of blocks from one of the said flows for which an examination of the error information concerning it contained in the said physical cell shows that it has the fewer errors.
According to another characteristic of the invention, it consists of using the said physical cells containing the error information as cells serving as markers for blocks or sets of blocks.
According to another characteristic of the invention, when the protection method according to the invention is applied to a telecommunication system in which the transmission of cells on each link takes place in cell-based mode, it consists of using, as cells serving as markers for blocks or sets of blocks, the so-called OAM F3 cells.
According to another characteristic of the invention, it consists:
if the two blocks or two groups of blocks belonging respectively to the two links are judged correct, of choosing indiscriminately either block or group of blocks,
if the block or group of blocks carried by the first link is judged correct whereas the block or group of blocks carried by the second link is judged incorrect, of selecting the block or group of blocks carried by the first link,
if, conversely, the block or group of blocks carried by the second link is judged correct whereas the block or group of blocks carried by the first link is judged incorrect, of selecting the block or group of blocks carried by the second link, and, finally
if the two blocks or two groups of blocks are judged incorrect, of using a selection process other than the block by block or group of blocks by group of blocks selection process.
For example, according to another characteristic of the invention, the said other selection process consists of performing a so-called cell by cell selection for each cell contained in each of the said two blocks. For example, the said cell by cell selection process consists of selecting the cell from one or other link which has the fewer errors in its header. Advantageously, the said cell by cell selection is based on the use of the syndrome of the so-called HEC field contained in the header of each cell.
According to another characteristic of the invention, the said cell by cell selection is based on the use, on the one hand, of the syndrome of the so-called HEC field contained in the header of the current cell and, on the other hand, of the syndrome of the so-called HEC field contained in the header of the following cell.
According to another characteristic of the invention, the said cell by cell selection consists of:
if the syndromes HEC(CN,1) and HEC(CN,2) are equal and if the same applies to the syndromes HEC(CN+1,1) and HEC(CN+1,2), choosing either cell indiscriminately,
if the syndromes HEC(CN,1) and HEC(CN,2) are equal whereas a comparison of the syndrome HEC(CN+1,1) with the syndrome HEC(CN+1,2) shows that the cell (CN+1,1) on the first link is less erroneous than the cell (CN+1,2) on the second link, selecting the cell CN,1,
if the syndromes HEC(CN,1) and HEC(CN,2) are equal whereas a comparison of the syndrome HEC(CN+1,1) with the syndrome HEC(CN+1,2) shows that the cell (CN+1,1) on the first link is more erroneous than the cell (CN+1,2) on the second link, selecting the cell CN,2,
if a comparison of the syndrome HEC(CN,1) with the syndrome HEC(CN,2) shows that the cell (CN,1) on the first link is less erroneous than the cell (CN,2) on the second link, selecting the cell (CN,1), and finally
if a comparison of the syndrome HEC(CN,1) with the syndrome HEC(CN,2) shows that the cell (CN,1) on the first link is more erroneous than the cell (CN,2) on the second link, selecting the cell (CN,2),
HEC(CN,X) being the function whose value represents the number of erroneous bits in the header of the cell of order N in the block under consideration and x being the number of the link on which the said cell is carried.
According to another characteristic of the invention, where the block by block or group of blocks by group of blocks selection is not used at a given moment, it consists of performing a cell by cell selection.
According to another characteristic of the invention, it consists, on the transmitter side, of inserting regularly, in each of the said flows, cells having an order number which is used for synchronizing the flows issuing respectively from the two links.